Encapsulation technology has been widely used in semiconductor devices, crystalline silicon solar cells, light emitting diodes (LEDs), organic light-emitting diodes (OLEDs), display screens, etc. Moreover, encapsulation films are used to adhere to the interior photovoltaic cell and to other layers in laminate structures and to protect the interior photovoltaic cell. Encapsulation films are usually made by polyolefin including ethylene-vinyl acetate (EVA) or polyolefin elastomer, which has been widely used for encapsulation.
EVA resin can be used as the sole material, plastic sheeting and hot melt adhesive. When used as a hot melt adhesive, the EVA resin that contains high weight percentage of VA (vinyl acetate) may be used, which may also has a low melting point, generally lower than 90° C. Before being used, the hot melt adhesive film, the EVA resin is made into a stick or an adhesive film that is convenient for users to process as needed. When the EVA resin contains VA having a weight percentage between 20 and 35 wt %, the EVA resin may possess excellent transparency greater than 90%, and may have desirable flexibility. Such EVA resin is ideally suited as a laminated film within double glaze or an encapsulation film for a solar module, which can buffer for the glass from being attacked or protect the very brittle solar cell chips behind the glass in the solar module. However, the EVA resin prepared to contain VA having a weight percentage between 20 and 35 wt % has a melting point between 60-80° C., lower than room temperature. It is hard for such EVA resin to maintain its dimensional stability and physical strength at room temperature for a long term. Such EVA resin has to be used after being crosslinked. In order to crosslink, thermo-crosslinking agent must be added to form an EVA resin film, which usually includes an organic peroxide, such as dicumyl peroxide (DCP), peroxy-2-ethylhexyl carbonate t-butyl ester (TBEC), etc. The EVA resin film with added thermo-crosslinking agent may be placed between the glasses in double glaze, or placed on both side of the solar cells behind the glass of solar module, vacuumed while being heated to be greater than 135° C. to melt the EVA resin to fill the gaps between the EVA resin film and the glass or between the EVA resin film and the solar cell. At the same time, EVA resin undergoes a crosslinking reaction by organic peroxide decomposition. In this stage, the crosslinking degree of EVA resin can reach 75-95%. The crosslinked EVA film is a thermoset material with elasticity without being melted and may permanently maintain the shape and strength. Usually the EVA film has not been crosslinked before used, the dimensional stability is poor, and it overflows from glass's edge after being heated, which may contaminate the equipment being used. When laying the colored and transparent EVA films up and down simultaneously, due to poor dimensional stability, the boundary of the colored and the transparent films may be unclear and interpenetrating.
A polyolefin elastomer (POE) resin refers to copolymer(s) of ethylene and butene, pentene, hexene or octene. It was first invented by Dow, which was copolymerized by octene and ethylene elastomer that possess relatively narrow molecular weight distribution and uniform short-branched chain distribution, using metallocene as catalyst. The elastomer crystalline region from ethylene chain in polyolefin elastomer is used as a physical crosslinking point, and butene, pentene, hexene, octene with long chain form amorphous rubber phase. As a result, the polyolefin elastomer has dual characteristics of rubber elasticity and thermoplasticity. The polyolefin elastomer is widely used in processing modified polyolefin such as modified polypropylene used in auto accessories, because it is well compatible with polyolefin especially polyethylene and polypropylene, and has excellent properties of weather resistance and no unsaturated bond simultaneously. The melting temperature of the polyolefin elastomer is low, usually between 50° C. - 70° C., there are almost no reports that polyolefin elastomer may be used by itself or polyolefin elastomer may be used as the main material. The polyolefin elastomer film made from the mixture of polyolefin elastomer and polyethylene compound by Dow, is served as a substitute of conventional EVA film in solar PV modules. The main component, polyolefin elastomer made from the mixture of polyethylene with high melt point and polyolefin elastomer compound with low melt point may have a melting point above 100° C. In a Chinese patent application CN103289582A, the polyolefin elastomer film can be prepared from the polyolefin elastomer by a reactive extrusion grafting step utilizing a graftable alkoxysilane-containing compound and a step adding organic peroxide. The polyolefin elastomer film may be crosslinked by organic peroxide decomposition by heating, providing heat-resistance. Due to very low melting point of the polyolefin elastomer, under the circumstance of adding polyethylene with high melting point or crosslinking agent, the melting point of the polyolefin elastomer is still very low. That the elastomer melt quickly during heating leads to inconvenience, meanwhile, low melting point causes high requirements for transportation and storage, which limits the use of the polyolefin elastomer.
Both POE film and EVA film have low melting points. The heating temperature for solar PV module's lamination is usually between 135° C. to 150° C., which is much higher than both melting points. Gradually the film melts in the lamination, and it is impossible to maintain moldability and stability, great changes may occur in the film's size and shape after lamination. For example: the two-layer film, whose size is smaller than glass, may overflow after lamination. In another example, when one of the two films is in color, after lamination the boundary is unclear and interpenetrating. The above problems have affected the final quality of the assembly or the production process.
It is important to improve the heat-resistant property of the EVA film or POE film and other polyolefin encapsulation film as hot-melt adhesive in the manufacturing process of encapsulation assembly to allow a clear interface of encapsulation.
Radiation crosslinking is a technique that uses crosslinking reaction between long chain polymers triggered by radiation. Radiation has two categories: one is ionizing radiation, which means that α (alpha), β (beta), γ (gamma), X and neutron rays, that can make a direct or indirect ionization (i.e., atoms or molecules gain or lose electrons and become ions); the other is non-ionizing radiation, such as visible light, ultraviolet light, sound radiation, heat radiation and low-energy electromagnetic radiation and so on. There is lack of binding force between molecular chains of the polymer, making it prone to deformation or damage when subjected to an external force and the ambient temperature, thus restricting its application. Crosslinking reaction forms binding sites such as chemical bonds between the polymer long chains so that the physical and chemical properties of the polymer are improved which is very effective means for polymer modification. During radiation crosslinking, there is no physical contact between the polymer and radiation generator, the shape of the polymer does not change before and after the reaction, but there is internal crosslinking reaction inside the polymer. Polymer product can be directly placed in the radiation for the crosslinking reaction. Further, crosslinking agent may be added into the polymer to promote efficiency of radiation crosslinking. However, non-ionizing radiation such as ultraviolet light has bad penetrability and limited cure depth, and moreover light initiator must be used for ultraviolet light curing. Therefore, it's difficult or impossible for partial crosslinking operation for polymer. However, radiation energy that can directly trigger polymer crosslinking reactions, such as β-rays, γ-rays, X-rays and etc. Use of radiation energy becomes more convenient, and achieves better results for partial crosslinking, since light initiator is not needed.
Radiation crosslinking is widely used in producing heat-shrinkable tube, and the plastics may have a shape memory capacity due to the crosslinking after radiation. Thus the plastic tube will return to the original shape. At room temperature the radiation-crosslinked plastic tube may be expanded, and then shrunk to its original shape while being heated. Another area in which it is commonly used is the production of automotive wire, radiation crosslinking can increase usage temperature of the automotive wire that has to be operated at a high temperature for vehicle engines.